The present invention relates generally to a solid state X-ray detector and more specifically to an improved scintillator sealing for a solid state X-ray detector.
The X-ray detectors have become essential in medical diagnostic imaging, medical therapy, and various medical testing and material analysis industries. One category of X-ray detectors uses scintillator materials to convert X-ray photons into visible-spectrum photons as part of the energy detection process. These scintillator materials are ionic salts such as CsI, which are hygroscopic. CsI is a crystalline material, with needle-shaped crystals. The crystals are oriented perpendicular to the plane of an adjacent glass substrate panel and act as short optical fibers to ensure that light photons originating in a crystal exit the crystal at its end and into an adjacent photodetector, rather than propagating within the CsI layer. The detector is sealed to prevent moisture from being absorbed into the scintillator. This moisture could adversely affect the crystal structure of the scintillator and degrade the image quality of the image detector. Additionally, the solid state electronics which convert the visible-spectrum photons to electrical signals in the image detector also should be protected from moisture to prevent their corrosion and consequent performance degradation.
A true hermetic seal, allowing effectively zero diffusion of moisture, generally requires an inorganic material such as metal or glass to act as the barrier to moisture. Organic materials, such as epoxy adhesives and sealants, do not offer true hermecity, but rather offer a low diffusion rate of moisture, which is dependent upon their formation, the path length required for moisture to penetrate through diffusion, and the quality of their adhesion to the surfaces they are sealing. Epoxy sealants and adhesives are referred to as semi-hermetic seals.
Current methods used to create a sent-hermetic seal use an epoxy sealant to attach a cover to the top layer of the image detector. The cover consists of a composite structural plate made of graphite fiber cloth in an epoxy matrix, with thin aluminum layers on one or both sides of the fiber cloth. The aluminum layers are positioned adjacent to the detector and provide a hermetic baffler over the detection surface area. This cover is bonded to the glass detector substrate with an epoxy seal, providing a semi-hermetic barrier at each edge of the cover. The X-ray image detector thus consists of a flat panel, with one face sealed by glass, one face sealed by aluminum, and the edges sealed by epoxy. Contained within the cover and detector layer are a scintillator and an Opticlad(trademark) layer. The Opticlad(trademark) layer consists of a plastic backing sheet with a layer of metal (typically silver or gold) and a layer of titanium oxide (TiO) and serves to reflect visible spectrum that would otherwise be wasted back to the diode layer of the detector where it is detected.
As the thickness of the scintillator layer is increased, the area over which the epoxy provides a semi-hermetic layer increases in direct proportion. Since the epoxy seal is not truly hermetic, this increases the probability of penetration by sufficient moisture to damage the detector. Also, application of the epoxy sealant required for a thicker scintillator layer is time-consuming.
It is therefore highly desirable to improve the method for sealing a scintillator for a solid state X-ray detector between the cover and substrate.
The present invention proposes several different methods by which to improve the hermetic sealing of the scintillator for a solid state X-ray image detector.
In one embodiment, a portion of the Opticlad(trademark) layer that is free of its TiO coating is extended. The metal outer layer of this portion of the Opticlad(trademark) layer is flexed towards and bonded to the glass substrate panel with an epoxy sealant, thereby creating a second semi-hermetic seal between the scintillator and outside moisture.
In another embodiment, an insulating layer is deposited onto the panel in the area to be used for the seal. Over that, a layer of metal that can be reflowed during laser welding is deposited. The metal layer of the Opticlad(trademark) layer is then laser welded to the metal layer on the top surface of the glass panel, thereby creating a second hermetic seal between the scintillator and outside moisture.
In a third embodiment, a metal frame is fabricated and sealed to the inner aluminum face of the graphite cover. The metal frame, preferably a metal alloy such as Kovar(copyright), has a length and width of the required seal, and of a rectangular section approximately equal to that of the scintillator. The metal frame replaces much of the volume of the epoxy seal, resulting in a smaller cross-sectional area of epoxy for moisture to diffuse through.
In a fourth embodiment, which also utilizes a metal frame, a metal such as nickel or gold that can be easily welded is deposited on the aluminum of the graphite composite cover. The metal frame is then welded or soldered directly to the deposited metal layer to create a cover layer with the metal frame attached, as compared to epoxy seal as in the third embodiment described above. This eliminates approximately one-half of the epoxy as used in the third embodiment, thus again reducing the exposed cross-sectional area of epoxy for moisture to diffuse through.
The fifth embodiment builds upon the principles of the third and fourth embodiments, and adds an insulating layer and metal layer that can be welded or soldered between the metal frame and glass substrate panel as well. In this method, the epoxy seal is completely eliminated, and thus the problem of moisture diffusion is also eliminated.
Other objects and advantages of the present invention will become apparent upon the following detailed description and appended claims, and upon reference to the accompanying drawings.